Air flow in enclosed spaces

ABSTRACT

A method of, and a system for promoting the flow of air from a lower location to an upper location, the method comprising: using heat from a heat source to provide a first heat store; and making a secondary use of heat from said heat source to power a heating device for heating an air conduit connecting said lower and upper locations; wherein said secondary use of heat from said heat source is controlled on the basis of an indicator of whether or not there is an excess of heat in the heat store.

The present invention relates to promoting the flow or air through anenclosed space, and to recovering heat from air rising from out of anenclosed space.

In one embodiment, the present invention relates to a method ofimproving passive summertime ventilation rates in buildings andincreasing the comfort of buildings in hot weather without using activeair conditioning.

In many buildings passive ventilation, driven by differences intemperature and wind pressure, is used to provide fresh air and removepollutants for occupant health, and to distribute heat or coolbuildings. In addition solar hot water heaters are often used to providehot water to buildings from the heat of the sun.

However, during hot, still days in summer, the airflow rates provided bypassive ventilation systems in buildings are often at their lowestalthough these are the times when higher ventilation rates are usuallyrequired for the cooling of occupants and the building fabric. Thereason for this is the weakness of the two driving factors at this timei.e. small temperature differences between inside and outside thebuilding and low wind speeds. This inability of passive systems torespond to hot still conditions makes them unsuitable for the cooling ofoccupants and buildings in summer.

Thermal solar panels absorb light and trap the energy as heat in thesolar panel. Fluid circulated through the panel transfers the heat to astore for later use. Due to the seasonality of solar energy in temperateclimates, sizing a solar panel to provide more than 50% of the hot waterload for a building becomes less economically efficient as you push tohigher fractions of solar energy. Over-sized solar panel areas increasesthe energy captured in winter, but the solar panels are idle for largeparts of the summer, spring and autumn, when they have already satisfiedthe water demand for the building.

U.S. Pat. No. 4,706,471 describes the use of heat energy from thermalsolar collectors to boost ventilation through increased buoyancy of theexhaust ventilation air. A number of drawbacks and/or deficiencies havebeen identified for the system described in U.S. Pat. No. 4,706,471.

It is an aim of the present invention to provide an effective techniquefor promoting the flow or air through an enclosed space.

It is another aim of the present invention to provide an effectivetechnique for recovering heat from heated air rising out of an enclosedspace.

According to one aspect of the present invention, there is provided amethod of promoting the flow of air from a lower location to an upperlocation, the method comprising: using heat from a heat source toprovide a first heat store; and making a secondary use of heat from saidheat source to power a heating device for heating an air conduitconnecting said lower and upper locations; wherein said secondary use ofheat from said heat source is controlled on the basis of an indicator ofwhether or not there is an excess of heat in the heat store.

According to another aspect of the present invention, there is provideda system for promoting the flow of air from a lower location to an upperlocation, the system comprising: a heat source; a first heat store forstoring heat from said heat source; and a heating device powered by asecondary use of heat from said heat source and for heating an airconduit connecting said lower and upper locations; wherein saidsecondary use of heat from said heat source is controlled on the basisof an indicator of whether or not there is an excess of heat in the heatstore.

In one embodiment, the secondary use of heat involves transferring heatfrom said heat source to said heating device via said heat store, and inanother embodiment, the secondary use of heat involves transferring heatfrom said heat source to said heating device other than via said heatstore.

According to another aspect of the present invention, there is provideda method of promoting the flow of air through a lower location intowhich a heated fluid is introduced via a heated fluid outlet, the methodincluding: directing said heated fluid to said heated fluid outlet via adevice for heating an air conduit connecting said lower location to anupper location and using a portion of the heat from said heated fluid topower said device.

According to another aspect of the present invention, there is provideda system for promoting the flow of air through a lower location intowhich a heated fluid is introduced via a heated fluid outlet, whereinthe system includes a device for heating an air conduit connecting saidlower location to an upper location, and wherein the system isconfigured such that said heated fluid is directed to said heated fluidoutlet via said device and a portion of the heat from said heated fluidis used to power said device.

According to another aspect of the present invention, there is provideda method of promoting the flow of air between a first lower location andan upper location including: heating at least a portion of an airconduit connecting the first lower location and the upper location topromote the flow of air up from the first lower location, wherein saidair conduit also connects said upper location to a second lowerlocation, and including providing a valve between said upper locationand said second lower location so as to impede the flow of air from saidfirst lower location to said second lower location via said air conduitwhilst allowing the flow of air from said second lower location to saidupper location.

According to another aspect of the present invention, there is provideda system for promoting the flow of air between a first lower locationand an upper location including: an air conduit connecting the firstlower location and the upper location; a device for heating at least aportion of said ari conduit to promote the flow of air up from the firstlower location, wherein said air conduit also connects said upperlocation to a second lower location, and wherein said system furtherincludes a valve between said upper location and said second lowerlocation so as to impede the flow of air from said first lower locationto said second lower location via said air conduit whilst allowing theflow of air from said second lower location to said upper location.

According to another aspect of the present invention, there is provideda method of ventilating an enclosed space containing a volume V m3 ofair, including promoting the flow of air through the enclosed space byheating an air conduit connected to an upper portion of said enclosedspace, wherein the air conduit has a cross-sectional area A m2, andwherein V>0.00015×A.

According to another aspect of the present invention, there is provideda system for ventilating an enclosed space containing a volume V m3 ofair, said system including an air conduit connected to an upper portionof said enclosed space, and a heating device for heat the air conduit topromote the flow of air through the enclosed space, wherein the airconduit has a cross-sectional area A m2, and wherein V>0.00015×A.

According to another aspect of the present invention, there is provideda device for controlling the intake of air into an enclosed space, thedevice including a first path for the intake of air into said enclosedspace via between the panes of a multi-glazed window, and a second pathfor the intake of air into said enclosed space other than via betweensaid panes of said multi-glazed window, wherein the device is configuredsuch that the opening of either of the first and second pathsautomatically closes the other of the first and second paths.

According to another aspect of the present invention, there is provideda method of recovering heat from air rising out of an enclosed space viaan air conduit, including transferring heat away from said air, whereinthe transfer of heat away from said air is actively controlled on thebasis of an indicator of the flow rate of air through said air conduit.

According to another aspect of the present invention, there is provideda system for recovering heat from air rising out of an enclosed spacevia an air conduit, including a device for actively controlling thetransfer of heat away from said air on the basis of an indicator of theflow rate of air through said air conduit.

According to another aspect of the present invention, there is provideda method of promoting the flow of air from a lower location to an upperlocation, the method comprising: using one or more fluid circuits totransfer heat from a heat source via a heat reservoir to air in an airconduit connecting said lower and upper locations, wherein the heatreservoir includes a tank of fluid having a cross-sectional area largerthan that of one or more fluid conduits constituting said one or morefluid circuits.

According to another aspect of the present invention, there is provideda system for promoting the flow of air from a lower location to an upperlocation, the system comprising one or more fluid circuits fortransferring heat from a heat source via a heat reservoir to air in anair conduit connecting said lower and upper locations; wherein said heatreservoir includes a tank of fluid of having a cross-sectional arealarger than that of one or more fluid conduits constituting said one ormore fluid circuits.

According to another aspect of the present invention, there is provideda method of promoting the flow of air from a lower location to an upperlocation, the method comprising: actively controlling the rate oftransfer of heat from a heat source to air in an air conduit connectingsaid lower and upper locations.

According to another aspect of the present invention, there is provideda system for promoting the flow of air from a lower location to an upperlocation, wherein the system includes a heat source, and the system isconfigured to actively control the rate of transfer of heat from theheat source to air in an air conduit connecting said lower and upperlocations.

According to another aspect of the present invention, there is provideda method of promoting the flow of air from a lower location to an upperlocation, the method comprising: using one or more fluid circuits totransfer heat from a heat source via a heat reservoir to air in an airconduit connecting said lower and upper locations, wherein the heatreservoir has a heat capacity greater than the combined heat capacity ofsaid one or more fluid circuits.

According to another aspect of the present invention, there is provideda system for promoting the flow of air from a lower location to an upperlocation, the system comprising one or more fluid circuits fortransferring heat from a heat source via a heat reservoir to air in anair conduit connecting said lower and upper locations; wherein said heatreservoir has a heat capacity greater than the combined heat capacity ofsaid one or more fluid circuits.

In one embodiment, ventilation or comfort is enhanced in buildings insummer by using a stored heat source to boost airflow rates throughbuoyancy effects.

In one embodiment, heat from solar processes is used as the heat source.

In one embodiment, ventilation boost is provided at a time separatedfrom the time of availability of the heat energy.

In one embodiment, domestic hot water demands as well as ventilationboost are satisfied without additional requirement for auxiliary heatfrom conventional heat sources.

In one embodiment, flow is diverted to an energy store for theventilation boost once domestic hot water needs are satisfied.

In one embodiment, a solar thermal store for the ventilation boostenergy is separated from a store for domestic hot water needs.

In one embodiment, a heat store for ventilation surrounds the airexhaust path.

In one embodiment, hot water feed is taken from a tap or shower througha heat exchanger in the passive stack to boost ventilation rates byincreasing the buoyancy of the air in a stack.

In one embodiment, reverse flow within stacks extracting from otherlocations (e.g. rooms) is prevented by forming a junction incorporatinga one-way valve between two stacks, thereby promoting boosted air flowwithin a subsidiary stack.

In one embodiment, a supply air window is modified for integration witha solar boosted whole house ventilation system whereby a means isprovided for the air inlet to the house provided by the window to enterthe house directly, by-passing the heat exchange channel between thewindow panes.

Embodiments of the invention will now be described in detail, solely byway of example, and with reference to the accompanying drawings inwhich:

FIG. 1 shows an embodiment of the present invention. It shows a domesticset-up with a roof mounted solar hot water panel connecting to a hotwater storage tank, which in turn interfaces to the exhaust of a passivestack ventilation system.

FIG. 2 shows another embodiment of the present invention. It shows across-section of a commercial building with a roof mounted solar waterheater connecting to a roof mounted hot water storage tank interfacingwith the air exhausting through a central atria.

FIG. 3 shows a heat exchanger for transferring heat into and out of thepassive stack.

FIG. 4 shows another embodiment of the present invention. It shows adetailed arrangement for storing solar derived heat energy, then laterusing it to boost a passive stack.

FIG. 5 shows another embodiment of the present invention; it shows adetailed arrangement for storing solar derived heat energy, then laterusing it to boost a passive stack, where the removal of excess energy isdetermined by the temperature in the store and does not cause theauxiliary heating to switch on.

FIG. 6 shows another embodiment of the present invention. It shows adetailed arrangement for storing solar derived heat energy, then laterusing it to boost a passive stack, with a separated thermal store forexcess solar energy.

FIG. 7 shows another embodiment of the present invention. It shows adetailed arrangement for storing solar derived heat energy, then laterusing it to boost a passive stack, with a simplified and separatedthermal store for excess solar energy.

FIG. 8 shows another embodiment of the present invention. It shows analternative buoyancy driven boost to a passive stack for the specialcase of a bathroom.

FIG. 9 shows a graph illustrating the relationship between flow ratesand buoyancy driven ventilation for different passive stack diameters.

FIG. 10 shows another embodiment of the present invention. It shows anon-return valve arrangement for the prevention of back-flow from astack with a solar boost into another ventilation path without such aboost.

FIGS. 11 and 12 show another embodiment of the present invention. Theyshow cross-sections of the base of a supply air window, modifiedaccording to an embodiment of the current invention with a by-pass. FIG.11 shows the supply air window in the heat recovery mode of operation;

FIG. 12 shows the window in by-pass mode.

FIG. 13 shows another embodiment of the present invention. It shows onearrangement that combines three modes of operation—solar heating of theheat store, a ventilation boost when excess energy is present in thestore and heat recovery from the exhaust air flow when conditions allow.

Referring to FIG. 1, a thermal solar panel 1, mounted on the roof 8,heats the hot water storage cylinder 2 via interconnecting pipes 3. Heatfrom the hot water cylinder circulates hot water to the heat exchanger 6in the passive stack 4 via interconnecting pipes 5. The exchange of heatinto the exhausting air increases the air flow 7 through the passivestack system 4.

It will be appreciated that this invention is not limited in applicationto domestic properties, and FIG. 2 shows such a situation. The roofmounted solar water heater 1 interfaces with a hot water storagefacility 2. Pipes 5 connect the storage facility with a heat exchanger 6situated in the central air exhaust channel or atria. Increased buoyancyof the air in the atria increases air flow 7 through the adjacenthabitable spaces 8.

The exchange of heat from heated liquid to the air in the passive stackcan be achieved for example by a device such as that shown in FIG. 3.The warm liquid is circulated to and from the hot water store either byactive pumping, or through passive thermo-syphon effect in pipe 5. Thepipes 5 are wound in intimate heat-conducting contact with a thermallyconductive inner wall 9. Air inside the wall 9 warms and expands,producing a buoyant force to increase the flow rate in the passivestack. The pipes 5 are preferably insulated on the outside with thermalinsulation 10 to undesirable heat loss to the environment. Fins or otherfeatures to increase surface area for heat exchange inside the tube canadvantageously increase the efficiency of the heat exchanger.

FIG. 4 shows an arrangement for the collection of solar hot water with amodification to allow thermal boost of passive ventilation. The hotwater store 2 consists of two heating zones V_(D) and V_(S). A coil ofpipe 12 is heated from a conventional heat source such as a boiler. Theposition of coil 12 is such that by convection it can heat the volumeV_(D) only. In one example, the volume is selected to be of similarmagnitude to the daily demand of the building, so that inlow-irradiation days, there is still sufficient hot water.

Coil 13, connected to the solar panel 1 via a pump 15 is located at thebottom of the cylinder, and in the right weather conditions can heat theentire cylinder volume V_(S) plus V_(D). If V_(D) is already hot, thenvolume V_(S) is always available to heat, and solar energy can becollected whenever it is available. In one example, the volume V_(S) isselected to be about 50% of V_(D), to take into account draw-offs of hotwater from the top of the cylinder during the day, by which the hot zoneof the cylinder shrinks, but remains as a layer floating on top of thecooler zone below which the solar coil 13 can heat.

In one variation, the system is modified by the addition of a circuit tothe passive stack heat exchanger 6, activated by a 3-way valve 16. Anelectronic controller is programmed to switch the valve 16 at apre-determined time to circulate from the solar coil 13 to the passivevent heat exchanger 6, so cooling the water in the cylinder below thetop of the coil by convection and heating the air in the passive stack 7and boosting air flow rates.

Alternatively, the fluid in the cylinder 2 could directly heat thepassive stack heat exchanger 6 via pipes attached to ports directly intothe cylinder, where there is no concern about fluid in this circuitentering the wholesome water supply, even where the circuit may bestatic for long periods in winter time.

The circulation to the passive stack could be active (by running thepump), or passive (driven by thermosyphon and eliminating energyoverhead associated with the pump).

The water in the cylinder zone V_(S) would only be cooled in a zonebelow the top of the coil since the cooling would be by convection,leaving a layer above the coil in V_(S) that is available to provide hotwater for the following day.

One possible drawback of this arrangement is that if the volume of V_(S)is increased to enable storage of sufficient energy for both theventilation boost and the following day's hot water needs, then oncertain days in spring and winter time when the ventilation boost is notrequired, the solar heating might produce a larger volume of tepid waterrather than a smaller amount of useful hot water. The boiler would fireunnecessarily, with the concomitant emission of greenhouse gases.

Another possible drawback with this arrangement is that if the householduses hot water from the cylinder (say in the early evening), then coldwater is introduced to the cylinder at the bottom. The hot solar-heatedwater floats on top. The solar coil is now in cold water and the excessenergy in the cylinder is not available for ventilation boost.

The arrangement in FIG. 5 avoids such drawbacks. The hot water store 2is allowed to exceed the demand temperature for hot water, when heatedby the solar coil 13. A blending valve at the outlet from the cylinderis provided to avoid dangerously hot water at the tap. A valve 19 isactuated to allow thermo-syphon flow from an indirect coil 24 near thetop of the store to the ventilation boost heat exchanger 6, once thedomestic hot water store 2 reaches a temperature greater than the settemperature for hot water. Control for the valve could be most simplyprovided by a thermostatic switch at the store. In this way, excesssolar heat energy is used to boost ventilation rates by raising thetemperature of the air in the exhaust path from the building andincreasing air flow through the tendency of hot air to rise. Thewithdrawal of heat energy from the store to boost ventilation rates doesnot reduce the temperature in the store to a temperature lower than theuseful temperature for domestic hot water, which might cause theactivation of an auxiliary heat source, such as a carbon fuel based heatsource (gas boiler or electric heating), and negate the energyefficiency benefits of the renewable energy based ventilation boost.

Since the ventilation boost coil 24 is located near the top of thestore, it can cool the whole depth of the store by convection. Thecontrol is set such that once the store has been cooled to a temperaturejust above the demand temperature for the store, the valve closes, andthe ventilation boost ends. In this way, the take off of energy forventilation boost does not inadvertently result in carbon fuelconsumption to heat the section of the cylinder V_(D).

The circulation could be augmented by an active pump to force thecirculation, also controlled by the same thermostatic means.

The arrangement in FIG. 6 also avoids such drawbacks. A separateventilation boost store 17 is provided for excess solar energy. Thisstore is dedicated to the storage of energy for ventilation boost. Adiverter valve 16 is actuated to divert flow from the solar loop to theventilation boost store once the domestic hot water store 2 issatisfied. Heat exchange to the ventilation boost store can be direct asshown in the figure, or indirect via a coil.

The ventilation boost store is connected to the passive stack heatexchanger 6 via pipes 18. Flow to and from the passive stack heatexchanger can be completely passive (driven by thermosyphon), passiveand controlled by a valve or pumped. In the latter two examples, theexact timing of the ventilation boost can be controlled electronically.In the former, the boost would tend to start later on in the day, andcontinue into the night since the rate of energy removal from theventilation boost cylinder would normally be much slower than the rateof addition from the solar panel.

There are benefits to achieving this separation in timing. Night timetemperatures are generally cooler than day time temperatures,particularly during the high pressure weather systems that produce thecombination of low wind speed and high ambient daytime temperatures thatcan diminish the effectiveness of passive ventilation systems. Storingthe energy collected during the day to boost night-time ventilationrates would decrease the temperature of the fabric of the building, andparticularly if coupled to building design incorporating high heatcapacity elements, would reduce the maximum daytime temperature thefollowing day.

A simplified arrangement of this design is illustrated in FIG. 7. Theventilation boost store 17 is integrated with the passive stack 4, witha heat conducting and thin walled tube passing through the ventilationboost store, and defining the ventilation exhaust path. Heat istransferred from the store to the air inside the tube by conductionthrough the wall.

The above-described techniques can be modified to run in reverse inwinter time. FIG. 13 illustrates one such arrangement to achieve this.Stale, warm air 7 leaving the house via the passive stack 4 could havesome of its energy reclaimed by the stack heat exchanger 6 andtransferred to the hot water store 2 via a system fluid circulated bypump 15 to heat exchange coil 13.

The arrangement further comprises two three-way valves 16 a, 16 b whichcan be configured to allow circulation between solar panel 1 and heatexchange coil 13 for collecting solar energy, coil 24 and passive stackheat exchanger 6 for ventilation boost and passive stack heat exchanger6 and coil 13 for heat recovery from the passive stack—all using thesame pump by setting the valves as indicated in the table.

The heat exchanger in the stack is configured to react dynamically tothe air flow in the stack, and only removes heat when the air flow rateis sufficient to accept this e.g. when the wind driven component of thestack flow is sufficient to make the buoyancy driven flow component ofless importance. One way to achieve this is to measure the flow rate inthe stack, for example with two spaced-apart pressure sensors, and toonly allow flow to transfer heat to the water store through the heatexchanger when the pressure difference between the sensors indicatesthat there is sufficient flow to accept heat removal.

Another application of buoyant boost for passive stack ventilationaccording to an embodiment of the present invention is illustrated inFIG. 8. The hot water feed to a bathroom or kitchen 21 is taken througha passive stack heat exchanger 6 before reaching outlets 20. In this waya buoyant boost for the passive stack 4 is achieved through increasedair flow 7 at the time when the room most requires a higher level of airextraction.

It has been found that the flow rate achieved by a buoyancy-driven boostfollows a non-intuitive relationship with the energy input. The benefitof designs that increase the energy input rapidly follow a curve ofdiminishing returns.

FIG. 9 illustrates this by way of a series of curves showing how thevolumetric flow rate in the stack varies with changes in the inputenergy. The series of curves are for stacks with increasingcross-sectional area from 50 mm diameter to 350 mm diameter in steps of50 mm. The dash line 30 shows a minimum useful flow rate for anight-time cooling strategy for a dwelling of 80 square metres floorarea.

We have discovered an important relationship between the cross-sectionalarea of the ventilation ducting and the flow rates induced by heatingthe exhaust gas, which serves to limit the effective area of the stackto a range above a minimum value.

It has been found that for a solar boosted ventilation stack to achievemeasurable improvements in the comfort of an enclosed space, theaggregate cross-sectional area of the heated stack in m² should begreater than 0.00015 times the volume of air in the enclosed space inm³.

A consequence of boosting passive stack ventilation is that stacksextracting from other rooms without the benefit of solar boost may,according to weather conditions, suffer from reverse flow. This is dueto the flow rate in the boosted stack being in excess of the flow thatthe other stack can achieve due to buoyancy forces alone, if thebuilding has been constructed to be air-tight.

FIG. 10 illustrates an arrangement designed to overcome this problem.Near the top of the boosted stack 4 a junction is made with the extractstack, without solar boost 23, from the other room. At the junction aone-way valve 22 allows air to be exhausted into the boosted stack andthe extract of air is promoted by the connection between the two. Theone way valve avoids any exchange of air between the two rooms byshutting across the end of the passive stack 23 to prevent reverse flow.

Some passively ventilated buildings employ so called “supply airwindows”, where air drawn into the building is by means of a flow paththat brings it between the panes 37 of a double glazed window. This hasthe beneficial effects of pre-heating the air in winter and minimisingthe experience of draughts, and lowers the effective U value of thewindow. According to another embodiment of the present invention, such awindow is modified by the addition of a by-pass valve to allow coolnight air directly into the house, without the need for opening thewindow, which many people consider to be an unacceptable security risk.

FIGS. 11 and 12 illustrate this embodiment. In the normal (heatrecovery) mode of operation, air from outside the building enters thewindow through inlet 36, and is drawn into the building through the gap34 between the window panes 37. In doing so, the air collects heat andenters the building at a higher temperature than the outdoortemperature. Since the heat flow escaping through the window iscollected and returned to the building, the effective insulation valueof the window is increased.

When cooling in summertime, it is desirable that the air enters thebuilding directly. A vent cover 31, is hinged 32 such that it can folddown to open a direct ventilation path to outside. The vent cover 32 isjoined to a secondary vent cover 33 by a linkage 38 which passes througha hole 35 in the window frame. The action of pulling the vent cover 31into the open position pulls the secondary vent cover 33 into a closedposition, allowing air to enter the house directly, and closing off airflow in the window gap 34. The effective insulation provided by thewindow is decreased, improving the cooling effect on the building.

The above-described techniques can apply in any situation where thepossibility for solar water heating, and passive ventilation exists e.g.domestic, commercial, educational and other types of buildings or fixedor mobile structures.

Advantages of one or more of the above-described embodiments are:

Using a renewable source of energy to cool buildings in summer saves theuse of carbon dioxide emitting electricity for air conditioning.

Solar thermal energy collection is significantly more cost effectivethan solar electric energy (PV)

The use of excess solar energy in summer to achieve a desirable effectin the building justifies the installation of a larger area of solarpanel, boosting the water heating benefits in Spring, Autumn and Winter.

The separation of the timing of energy collection and effect enablesnight-time ventilation boost which can cool the fabric of the buildingmore effectively than a daytime boost.

The applicants draw attention to the fact that the present invention mayinclude any feature or combination of features disclosed herein eitherimplicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any definitions set out above.

Furthermore, in view of the foregoing description it will be evident toa person skilled in the art that various modifications may be madewithin the scope of the invention. For example, each of the techniquesillustrated in FIGS. 8, and 10 to 12 can be used individually ortogether in combination with any of the techniques illustrated in FIGS.1, 2, 4 to 7 and 13.

1. A method of promoting the flow of air from a lower location to anupper location, the method comprising: using heat from a heat source toprovide a first heat store; and making a secondary use of heat from saidheat source to power a heating device for heating an air conduitconnecting said lower and upper locations; wherein said secondary use ofheat from said heat source is controlled on the basis of an indicator ofwhether or not there is an excess of heat in the heat store.
 2. A methodaccording to claim 1, wherein said secondary use of heat comprisestransferring heat away from said heat store to said heating device.
 3. Amethod according to claim 1, wherein said heating device includes asecond heat store, and from which heat can be controllably transferredto said air conduit.
 4. A method according to claim 3, further includingactively controlling the timing of the transfer of heat from said secondheat store to said air conduit.
 5. A method according to claim 1,wherein said heating device includes an inlet for receiving hot fluidfrom said heat store and/or said heat source and an outlet for returningfluid to said heat store and/or said heat source, and wherein theheating device has a part for transferring heat to the air conduit whichdefines a flow path for fluid having a cross-sectional area greater thanthat of the inlet and outlet.
 6. A method according to claim 1,including triggering said secondary use of heat from said heat sourcewhen an indicator of the temperature of the heat store exceeds apredetermined value, and continuing said secondary use until saidindicator no longer exceeds said predetermined value.
 7. A methodaccording to claim 1, wherein the heat source is a solar device forcapturing heat from solar energy.
 8. A method according to claim 1,wherein the heat store is a source of hot water in a first storagevessel.
 9. A system for promoting the flow of air from a lower locationto an upper location, the system comprising: a heat source; a first heatstore for storing heat from said heat source; and a heating devicepowered by a secondary use of heat from said heat source and for heatingan air conduit connecting said lower and upper locations; wherein saidsecondary use of heat from said heat source is controlled on the basisof an indicator of whether or not there is an excess of heat in the heatstore.
 10. (canceled)
 11. (canceled)
 12. (canceled)
 13. A methodaccording to claim 1, wherein said air conduit also connects said upperlocation to a second lower location, and including providing a valvebetween said upper location and said second lower location so as toimpede the flow of air from said first lower location to said secondlower location via said air conduit whilst allowing the flow of air fromsaid second lower location to said upper location.
 14. A systemaccording to claim 9, wherein said air conduit also connects said upperlocation to a second lower location, and wherein said system furtherincludes a valve between said upper location and said second lowerlocation so as to impede the flow of air from said first lower locationto said second lower location via said air conduit whilst allowing theflow of air from said second lower location to said upper location. 15.(canceled)
 16. A use of a method according to claim 1 for ventilating anenclosed space containing a volume V m3 of air, wherein the air conduithas a cross-sectional area A m2, and wherein V>0.00015×A.
 17. A systemaccording to claim 9, wherein said lower location defines an enclosedspace containing a volume V m3 of air, wherein the air conduit has across-sectional area A m2, and wherein V>0.00015×A.
 18. (canceled) 19.(canceled)
 20. (canceled)
 21. (canceled)
 22. A method according to claim1, further including transferring heat away from air flowing through theair conduit to the heat store when the flow rate of air from the lowerlocation to the upper location is above a predetermined minimum flowrate even without using heat from the heat source, and activelycontrolling the transfer of heat away from the air to the heat sourcesuch that it does not reduce the flow rate of air through said airconduit to below said predetermined minimum flow rate.
 23. A systemaccording to claim 9, further including a device for activelycontrolling the transfer of heat away from said air on the basis of anindicator of the flow rate of air through said air conduit.
 24. A methodaccording to claim 30 comprising: using one or more fluid circuits totransfer heat from a heat source via a heat reservoir to air in an airconduit connecting said lower and upper locations, wherein the heatreservoir includes a tank of fluid having a cross-sectional area largerthan that of one or more fluid conduits constituting said one or morefluid circuits.
 25. (canceled)
 26. A method according to claim 30comprising: actively controlling the rate of transfer of heat from aheat source to air in an air conduit connecting said lower and upperlocations.
 27. (canceled)
 28. A method according to claim 30 comprising:using one or more fluid circuits to transfer heat from a heat source viaa heat reservoir to air in an air conduit connecting said lower andupper locations, wherein the heat reservoir has a heat capacity greaterthan the combined heat capacity of said one or more fluid circuits. 29.(canceled)
 30. A method of promoting the flow of air from a lowerlocation to an upper location, the method comprising: (a) using one ormore fluid circuits to transfer heat from a heat source via a heatreservoir to air in an air conduit connecting said lower and upperlocations, wherein the heat reservoir includes a tank of fluid having across-sectional area larger than that of one or more fluid conduitsconstituting said one or more fluid circuits; or (b) activelycontrolling the rate of transfer of heat from a heat source to air in anair conduit connecting said lower and upper locations; or (c) using oneor more fluid circuits to transfer heat from a heat source via a heatreservoir to air in an air conduit connecting said lower and upperlocations, wherein the heat reservoir has a heat capacity greater thanthe combined heat capacity of said one or more fluid circuits.